Minimally- patterned, thin-film semiconductor devices for display applications

ABSTRACT

A thin-film transistor array comprises at least first and second transistors. Each of the first and second transistors include a shared silicon layer, i.e., an active layer, having a thickness less than approximately 40 nm. The shared silicon layer extends continuously between the first and second transistors. The silicon layer may consist of unpatterned silicon. Heavily doped material may not be required at metal-silicon contact interfaces.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims the benefit under 35 USC §119(e)of United States Provisional Patent Application Ser. No. 60/218,490,filed Jul. 14, 200, the entire contents of which are incorporated hereinby reference. The present application is filed simultaneously withUnited States Patent Application entitled “Fabrication of ElectronicCircuit Elements Using Patterned Semiconductor Layers”, attorney docketnumber INK-100, the entire contents of which are incorporated herein byreference.

FIELD OF THE INVENTION

[0002] The present invention relates generally to electronic displaysand methods of manufacturing the electronic displays, and moreparticularly to, semiconductor devices for electronic displayapplications and methods of manufacturing the semiconductor devices.

BACKGROUND OF THE INVENTION

[0003] Some encapsulated, particle-based displays offer a useful meansof creating electronic displays. There exist many versions ofencapsulated particle-based displays including encapsulatedelectrophoretic displays, encapsulated suspended particle displays, androtating ball displays.

[0004] Encapsulated, particle-based displays can be made highlyreflective, bistable, and optically and electrically efficient. Toobtain a high-resolution display, however, individual pixels of adisplay must be addressable without interference from adjacent pixels.One way to achieve this objective is to provide an array of nonlinearelements, such as transistors or diodes where each transistor or diodeis associated with each pixel. An addressing electrode is connected toeach pixel through the transistor or the diode.

[0005] The processes for manufacturing active matrix arrays of thin-filmtransistors and diodes are well established in the display technology.Thin-film transistors, for example, can be fabricated using variousdeposition and photolithography techniques. A transistor includes a gateelectrode, an insulating dielectric layer, a dielectric layer and sourceand drain electrodes. Application of a voltage to the gate electrodeprovides an electric field across the dielectric layer, whichdramatically increases the source-to-drain conductivity of thesemiconductor layer. This change allows for electrical conductionbetween the source and the drain electrodes. Typically, the gateelectrode, the source electrode, and the drain electrode are patterned.In general, the semiconductor layer is also patterned in order tominimize stray conduction (i.e., cross-talk) between neighboring circuitelements.

[0006] Liquid crystal displays commonly employ amorphous silicon(“a-Si”), thin-film transistors (“TFT”) as switching devices for displaypixels. Such TFTs typically have a bottom-gate configuration. Within onepixel, a thin-film capacitor typically holds a charge transferred by theswitching TFT. Thin-film transistors can be fabricated to provide highperformance. Fabrication processes, however, can result in significantcost.

[0007] Referring to FIG. 1, a thin-film transistor, having typicalcontact structures, and a capacitor are illustrated in cross-section.The transistor and capacitor include bottom electrodes 153, 155, asilicon nitride (“SiN”) dielectric layer 154, an a-Si layer 156, an n⁺a-Si contact layer 158, drain and pixel electrodes 159, and capacitortop electrode 192. The a-Si layer 156, the n⁺ a-Si contact layer 158 andthe electrodes 159 are all patterned layers.

[0008] The n⁺ a-Si contact layer 158 is typically 40 nm thick andprovides an ohmic contact between the a-Si layer 156 and the electrodes159. The patterning of the n⁺ a-Si layer 158 generally requiresoveretching to assure complete removal of the n⁺ a-Si contact layer 158along the channel portion of the a-Si layer 156. Thus, a portion of thea-Si layer 156 is removed during this overetch step. Hence, the a-Silayer 156, as-deposited, is traditionally 160 nm or more in thickness.

[0009] The high cost of manufacturing thin-film transistors results inpart from patterning steps, which typically require the use of expensivephotolithography equipment and masks, coating steps, and etching steps.An a-Si layer is typically patterned to leave islands of semiconductormaterial and thereby reduce leakage currents. Formation of thestructures illustrated in FIG. 1 might require three lithography stepsand four etching steps. Trends toward making higher performance devicesmake precision patterning even more important and manufacturing costeven greater.

[0010] Certain electronic devices, however, require low cost rather thanhigh performance components. For such devices, it remains desirable tohave means to obtain better yield and lower cost of manufacturing.

SUMMARY OF THE INVENTION

[0011] The invention is based in part on the realization that a low costdisplay device transistor array having a shared, very thin a-Si layermay support good image resolution while providing tolerable leakagecurrents. The invention features electronic circuits that have a lowermanufacturing cost and methods of making electronic circuits thatinvolve simpler processing steps. The circuits are particularly usefulfor addressing display media in a display device.

[0012] In a preferred embodiment, the circuits comprise thin-filmtransistors (“TFT”) that share a continuous semiconductor layer, hereinreferred to as the “active layer”, that mediates current between sourceand drain of each transistor in an array of transistors. Thesemiconductor layer may be unpatterned. The layer may be continuous intwo dimensions, e.g., it may be shared by, and continuous between, TFTsin a two-dimensional array. The display medium controlled by thecircuits may be tolerant of leakage currents that flow through thecontinuous semiconductor layer. Devices of the invention are ofparticular use in the fabrication of electrophoretic displays.

[0013] In a preferred embodiment, the continuous semiconductor layer isa very thin layer, for example, most effective at less thanapproximately 40 nm in thickness, and supports the active regions for anarray of TFTs. Prior art transistors typically require deposition ofheavily doped silicon material, e.g., n⁺ a-Si, at the interface betweenmetal-to-silicon contacts. The heavily doped material assists formationof an ohmic rather than a Schottky contact. In contrast, variousembodiments of TFTs of the invention require no heavily doped material,e.g., n⁺ a-Si, at contact interfaces, e.g., the interface of thesemiconductor layer to a source metal electrode or a drain metalelectrode.

[0014] Embodiments that require no n⁺ a-Si material at interface providenumerous potential advantages over the prior art. For example, TFTarrays may be fabricated with no patterning of a semiconductor layer,i.e. the active layer, or deposition and patterning of a heavily dopedsemiconductor layer at contact interfaces. This may eliminate aphotolithographic step and a dry etching step, in addition toeliminating formation of a heavily doped layer at metal contactinterfaces.

[0015] Elimination of n⁺ a-Si from fabrication may further eliminateassociated costs due to a related deposition chamber and hazardsentailed by use of highly toxic and flammable PH₃ gas. Relatedelimination of a dry etch step permits use of all-wet fabrication,further reducing fabrication costs.

[0016] Moreover, the above features of the invention provide increasedfabrication throughput. Use of a thinner semiconductor active layerreduces semiconductor deposition time. Elimination of a heavily dopedsemiconductor layer, and elimination of patterning of the semiconductoractive layer, further increase fabrication throughput. In someembodiments, a SiN layer, an a-Si layer and a metal 2 layer aredeposited in the same deposition system, again improving manufacturingthroughput.

[0017] The invention may provide improved fabrication yield, due tosimplified processing. Moreover, some embodiments may utilize aroll-to-roll substrate fabrication process. Continuous deposition of thegate dielectric, a-Si, and source-drain electrode metal without a breakin vacuum, for example, as well as an all-wet etching process, arecompatible with roll-to-roll processing.

[0018] Though use of an unpatterned active layer may increase deviceleakage, appropriate design and application of a TFT array may provideacceptable performance. The spacing between transistors may be selectedto obtain acceptable leakage currents. The geometry of the transistorsmay be selected to obtain an acceptable leakage current between a firstdata line and a second data line. Alternatively, the spacing between thefirst data line and a first pixel electrode may be chosen to provide anacceptable leakage current between the first data line and the firstpixel electrode. Use of a very thin active layer may permit closerpacking of devices than otherwise possible.

[0019] Accordingly, in a first aspect, the invention features athin-film transistor array that includes at least first and secondtransistors. Each of the first and second transistors include a sharedsilicon layer, i.e., an active layer, having a thickness less than 40nm. The shared silicon layer extends continuously between the first andsecond transistors. Each transistor further has a source electrode and adrain electrode spaced from the source electrode, both in direct contactwith the silicon layer. Each transistor also has a gate electrodedisposed adjacent to the silicon layer.

[0020] The silicon layer may consist of unpatterned silicon. Hence, thesilicon may be a continuous film of material, use of which may reducethe number of process steps involved in manufacturing the transistorarray. The silicon layer may consist of amorphous silicon, and thesilicon layer may be undoped.

[0021] Use of an extremely thin silicon layer may obviate a need for ahighly doped layer of material lying between the extremely thin siliconlayer and source and drain contacts. Prior art thin-film transistorarrays typical require the highly doped layer to provide a good ohmiccontact.

[0022] The first transistor may be a bottom gate or a top gatetransistor. The first transistor may include a first pixel electrode ofan electronic display, the first pixel electrode in communication withthe source electrode of the first transistor, and the drain electrode ofthe first transistor is in communication with a first data line of theelectronic display. A distance between the first pixel electrode and thefirst data line may be selected to provide an acceptable leakage currentbetween the first pixel electrode and the first data line. Though use ofan unpatterned silicon layer may lead to increased leakage current,transistor geometry may be adjusted to reduce leakage to tolerablelevels.

[0023] Different geometrical aspects of a transistor array may beselected to reduce leakage. The distances between a pixel electrode andeach of the adjacent data lines may be selected to provide an acceptableleakage current between the first data line and the second data line. Atleast one of the first data line, the second data line, the firsttransistor and the first pixel electrode may have a geometry selected toprovide an acceptable leakage between the first data line and the seconddata line.

[0024] In a second aspect, the invention features an electronic display.The display includes a display medium, a first pixel electrode and asecond pixel electrode adjacent to the display medium, and a firstthin-film transistor and a second thin-film transistor in respectiveelectrical communication with the first pixel electrode and the secondpixel electrode, and comprising a shared continuous amorphous siliconlayer that has a thickness less than 40 nm and provides channels for thefirst thin-film transistor and the second thin-film transistor.

[0025] The electronic display may include any of a variety of displaymedia, for example, an electrophoretic medium. An electrophoretic mediummay have at least one type of particle and a suspending fluid, and maybe encapsulated.

[0026] The electronic display may further include a light blocking layerprovided adjacent to the silicon layer. As described above, transistorgeometrical features may be adjusted to reduce leakage currents.

[0027] In a third aspect, the invention features a method ofmanufacturing an array of thin-film transistors. The method includes thesteps of providing a substrate, forming adjacent to the substrate anunpatterned silicon layer having a thickness less than 40 nm. At leastone patterned drain electrode is formed for each of the transistors.Drain electrodes are formed in direct contact with the unpatternedsilicon layer. At least at least one patterned source electrode isprovided for each of the transistors. The source electrodes are indirect contact with the unpatterned silicon layer. At least one gateelectrode is provided for each of the transistors. The gate electrode isdisposed adjacent to the unpatterned silicon layer.

[0028] A dielectric layer may be formed adjacent to the at least onegate electrode. Forming the dielectric layer, forming the unpatternedsilicon layer and forming the metal layer which will, after patterning,form the source and drain electrodes may occur during one visit of thesubstrate inside a single deposition chamber. Providing a substrate mayinclude unwinding the substrate from a first roll and winding thesubstrate onto a second roll.

[0029] The method may further include providing a first pixel electrodeof an electronic display in communication with the source electrode ofthe first transistor, and providing a first data line of the electronicdisplay in communication with the drain electrode of the firsttransistor. The method may further include providing a second pixelelectrode of an electronic display in communication with the sourceelectrode of the second transistor and providing a second data line ofthe electronic display in communication with the drain electrode of thesecond transistor.

[0030] Various geometrical parameters may be adjusted to provideacceptable leakage currents. Geometrical parameters include the shapesof features and the spacings between features. Features include, forexample, the data lines, the transistors and the pixel electrodes.

[0031] Forming may include mask steps consisting of a first mask stepand a second mask step. At least one patterned gate electrode is formedin the first mask step, and at least one drain and one source electrodeis formed the second mask step. Hence some embodiments include exactlytwo mask steps. (As in many prior art processes, and additional maskstep may be required to form contacts adjacent the edges of thedisplay.)

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The foregoing and other objects, features and advantages of thepresent invention, as well as the invention itself, will be more fullyunderstood from the following description of preferred embodiments, whenread together with the accompanying drawings, in which:

[0033]FIG. 1 shows a diagrammatic cross-sectional view of a prior artTFT and capacitor.

[0034]FIG. 2 shows a cross-sectional view of an array of thin-filmtransistors according to one embodiment of the present invention.

[0035]FIG. 3 shows a top view of one embodiment of an electronicdisplay, with the display medium removed.

[0036]FIG. 4 illustrates locations of resistive leakage paths for thedisplay of FIG. 3.

[0037]FIG. 5a shows an underneath plan view of an embodiment of athin-film transistor with the substrate omitted.

[0038]FIG. 5b shows a diagrammatic cross sectional view that correspondsto the transistor embodiment shown in FIG. 5a.

[0039]FIG. 6 shows a graph of drain current versus gate voltage for asample of a two-mask transistor of the type shown in FIG. 5a.

[0040]FIG. 7 shows a cross-sectional view of an array of thin-filmtransistors according to one embodiment of the present invention.

[0041]FIG. 8 shows a cross-sectional view of an array of thin-filmtransistors according to one embodiment of the present invention.

[0042]FIG. 9 shows a cross-sectional view of an array of thin-filmtransistors according to one embodiment of the present invention.

[0043]FIG. 10 shows a cross-sectional view of an array of thin-filmtransistors according to one embodiment of the present invention.

[0044]FIG. 11 shows a cross-sectional view of an electronic displayaccording to one embodiment of the present invention.

[0045]FIG. 12 shows a diagrammatic cross-sectional view of a transistorand capacitor of an array, according to one embodiment of the invention.

[0046]FIG. 13 shows a graph of drain current versus gate voltage for asample transistor of an embodiment with a 10 nm thick a-Si layer.

[0047]FIG. 14 shows a graph of drain current versus drain voltage forthe sample transistor of FIG. 13.

[0048]FIG. 15 shows a graph of transient voltage switching and holdingof a sample transistor array.

[0049]FIG. 16a shows a diagrammatic cross-sectional view of anelectronic display according to one embodiment of the present invention.

[0050]FIG. 16b shows a diagrammatic cross-sectional view of anelectronic display according to one embodiment of the present invention.

[0051]FIG. 16c shows a diagrammatic cross-sectional view of anelectronic display according to one embodiment of the present invention.

[0052]FIG. 16d shows a diagrammatic cross-sectional view of anelectronic display according to one embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0053] In one aspect, the invention features minimally-patternedsemiconductor devices for display applications. In a preferredembodiment, the semiconductor devices are an array of thin-filmtransistors. An array of TFTs may include a continuous a-Si layer ofapproximately 40 nm or less in thickness, preferably 30 nm or less, mostpreferably 20 nm or less, without heavily doped a-Si at metal contactinterfaces. In the following, general considerations of transistor arraydesign and leakage currents will be discussed. Simplified arrays fordisplays that can tolerate leakage in a variety of semiconductormaterials that provide for TFT active layers, are described. Then,arrays employing very thin a-Si for the active layer are described. Afinal section describes some display media that may be used with TFTarrays in the fabrication of a display.

[0054] Referring to FIG. 2, an array of transistors 10 includes asubstrate 12, a gate electrode 14 for each transistor provided adjacentto the substrate 12, a gate dielectric layer 16 provided adjacent to thesubstrate 12 and the gate electrodes 14, a semiconductor layer 18provided adjacent to the gate dielectric layer 16, and a sourceelectrode 20 and a drain electrode 22 for each transistor providedadjacent to the semiconductor layer 18. The sizes of the electrodes 20,22 may vary in various transistor designs.

[0055] For fabrication of thin-film transistors, the substrate 12 maybe, for example: a silicon wafer; a glass plate; a steel foil; or aplastic sheet (for example a polyimide sheet). The gate electrodes 14,for example, can be any conductive material such as metal or conductivepolymer. The materials for use as the semiconductor layer 18, forexample, can be inorganic materials such as amorphous silicon orpolysilicon. Alternatively, the semiconductor layer 18 can be formed oforganic semiconductors such as: polythiophene and its derivatives;oligothiophenes; and pentacene. In general, any semiconductive materialuseful in creating conventional thin film transistors can be used inthis embodiment. The material for the gate dielectric layer 16 can be anorganic or an inorganic material. Examples of suitable materialsinclude, but are not limited to, polyimides, silicon dioxide, and avariety of inorganic coatings and glasses. The source and drainelectrodes 20, 22 may be made of any conductive material such as metalor conductive polymer.

[0056] The array of transistors illustrated in FIG. 2 can bemanufactured using any one of many appropriate methods. For example,vacuum based methods such as chemical vapor deposition, evaporation, orsputtering can be used to deposit the materials necessary to form thetransistor and thereafter the deposited material can be patterned.Alternatively, wet printing methods or transfer methods can be used todeposit the materials necessary to form the transistors.

[0057] The array of transistors described in reference to FIG. 2 can beused for addressing an electronic display. This embodiment is applicableto a variety of electronic displays, including: electrophoreticdisplays; liquid crystal displays; emissive displays (including organiclight emitting materials); and, rotating ball displays. For liquidcrystal displays, error limits place a demand on the time-averagedsquare of the voltage across the pixel. For current-driven, emissivedisplays, the acceptable tolerance in voltage variation will depend uponhow emission varies with current through the pixel. In general, displaytypes that have switching elements with a threshold associated withswitching, rather than a gradual change in optical state, will be moretolerant of errors.

[0058] In the embodiment of FIG. 2, while the electrodes 14, 20, 22(i.e., gate electrode, source electrode and drain electrode) arepatterned, the semiconductor layer 18 is not, resulting in significantreduction in processing efforts and cost. This circuit design canexhibit cross-talk between adjacent transistors that reside in rows andcolumns of transistors in an array. The degree of cross-talk, however,can be reduced to a level that is acceptable for some applications.

[0059] For some applications, a degree of cross-talk can be tolerated.For example, if only a few gray level states of a display are addressed,then small stray voltages may not significantly affect the overallappearance of the display. In addition, if the display is designed formoderate resolution, then neighboring circuit elements will be far apartfrom each other, reducing the degree of cross-talk.

[0060] In general, cross-talk errors are noticeable in displays only ifthey cause unwanted optical changes in pixel areas surrounding any oneparticular pixel element. In particular, if a pixel has only twopossible switching states, i.e. either dark or light, then smalldeviations in the electronic signal due to cross-talk may notsubstantially change the optical appearance of the pixel. Whenintermediate optical states, i.e. gray levels, are being addressed,however, the display pixel elements will be more sensitive to errors.Here, it is more likely that an incorrect gray level will be displayedby a pixel.

[0061] Depending on a particular display type and application, a smalleror larger error tolerance can be preferred. A monochrome display, forexample, may be able to tolerate leakage currents in excess of 10%,whereas a 256-level display would typically require a much lower leakagelevel of approximately 0.2%. A tolerance level may be estimated bydividing 100% by twice the number of gray levels, because typically theleakage current should not cause more than one-half a gray level error.In a preferred embodiment, the display incorporates pixels with alimited number of gray levels. In this case, a given pixel is lesssensitive to cross-talk induced voltage errors because it is switchedbetween a limited number of optical states.

[0062] For a particular display, the acceptable leakage will depend onthe extent of error in the electrical signal seen by a pixel and howthat affects the optical state of the pixel. This will depend on thedisplay medium. For displays that depend on a DC signal to switch,including electrophoretic and rotating ball displays, and ferroelectricliquid crystal displays, the switching electronic signal depends on boththe magnitude and duration of the voltage applied. The acceptableleakage corresponds to a maximum tolerable error in the optical state ofa display pixel.

[0063] An array of transistors with acceptable cross-talk can beprepared by following the design rules provided herein in reference toFIG. 3, which illustrates a plan-view of the conductive leads and theelements for driving a display. An array comprises: data lines 30, 32;select lines 36, 46; and pixel electrodes 34, 38, 40, 42. To address apixel electrode 34, 38, 40, 42, voltages are applied to appropriate datalines 30, 32 and select lines 36, 46. For example, to address particularpixel electrode 34, voltages are applied to data line 30 and select line36. Changes in the optical characteristics of a display element areachieved by addressing a pixel electrode 34, 38, 40, 42 that isassociated with the display element.

[0064] A preferred embodiment includes two design criteria for aproperly functioning display. First, referring to FIG. 3, the resistiveleakage between neighboring data lines 30, 32 must be small such thatthe voltage applied to each data lines 30, 32 can be controlled towithin desired tolerances by the associated driver elements. A resistiveleakage between neighboring data lines is too large when it leads tounacceptable resistive voltage drops in the driver circuit or in thedata lines. Second, the current leakage to the pixel electrode from eachof the two adjacent data lines 30 or 32, when the correspondingtransistor is switched “off”, must be sufficiently small to avoidunacceptable optical artifacts.

[0065] Many video displays produce video output by periodically updatingstill images presented in rapid succession at some frame rate. Eachimage is presented for a period of time, i.e., a frame time. When theoptical character is determined primarily by the time-varying voltageprofile on the pixel electrode, such as for electrophoretic andtwisted-nematic displays, the impact of current leakage on the voltageprofile preferably is sufficiently small during the frame time. A pixelvoltage preferably does not change by an unacceptable amount during aframe time because a pixel preferably maintains a given optical stateduring this interval of time.

[0066] For example, a large current leakage between the data line 30 andpixel electrode 34 may cause an unintended shift in the pixel voltage,thus changing the optical state of that pixel during the presentation ofa single image by a display. In a display using emissive material, suchparasitic leakage currents can cause unwanted light emission from thepixel.

[0067] The following discussion illustrates how the above described twodesign criteria can be calculated. Since the semiconductor layer is muchthinner than the lateral gaps between the electrical elements,resistance calculations can employ a thin-film approximation.

[0068] The First Design Criterion

[0069] The conduction between adjacent data lines 30, 32 is greatlyfacilitated by the presence of a column of pixel electrodes 34, 40. Anefficient conduction path can be approximated as follows. Current canleak from the first data line 30 to the adjacent column of pixelelectrodes 34. Note that the display has a first row of pixel electrodes34, 38 and a second row of pixel electrodes 40, 42. More generally, ifthere are N rows in a particular display, N being an integer, then thereare N conduction paths in parallel between adjacent data lines 30, 32and the resistive pathway between adjacent data lines 30, 32 can beapproximated by the resistive elements shown in FIG. 4.

[0070] Referring to FIG. 4, RTFT is the resistance between the firstdata line 30 and the pixel electrode 34 through the thin-film transistorchannel in the “off” state, R₁ is the resistance across the gap betweenthe first data line 30 and the pixel electrode 34 and R₂ is theresistance across the gap between the pixel electrode 34 and the seconddata line 32. The resistive pathway provided directly between adjacentdata lines 30, 32 along the region between neighboring pixel electrodes34, 40 can be neglected as being insignificant in comparison to thepathway provided by the pixel electrodes 34, 40, i.e. the pixelelectrodes 34, 40 are good conductors. Using this model, the resistanceacross adjacent data lines 30, 32 (R_(dd)) can be expressed as:

[0071] where:${R_{dd} = {\frac{1}{N}\left\lfloor {R_{2} + \left( {\frac{1}{R_{TFT}} + \frac{1}{R_{1}}} \right)^{- 1}} \right\rfloor}},\text{where:}$${R_{TFT} = \frac{\rho \quad L}{W\quad h}};$${R_{1} = \frac{\rho \quad L_{1}}{\left( {Y_{p} - W} \right)h}};$$R_{2} = {\frac{\rho \quad L_{2}}{Y_{p}h}.}$

[0072] N is the number of rows of pixel electrodes, ρ is the bulkresistivity of the semiconducting layer, L is the distance betweensource and drain electrodes, L₁ is the distance between a data line andthe adjacent pixel electrode, L₂ is the distance between the pixelelectrode and the neighboring data line, Y_(p) is a width of a pixelelectrode, W is the channel width, and h is the thickness of thecontinuous semiconductor layer.

[0073] A properly functioning display will have a resistance betweenadjacent data lines 30, 32 that is much greater than the resistancebetween the data lines 30, 32 and the voltage source (R_(d)). In theapproximation where the thin-film transistor channel width is muchsmaller than the pixel width (Y_(p)), this condition can be achieved bya display which obeys the two inequalities:

R ₁ +R ₂ >>NR _(d)

[0074] and

R _(TFT) >>NR _(d)

[0075] The data line also should not charge up an adjacent pixel whilethe select line is off (row unselected). This demand can be translatedas:

R _(TFT) >>R _(p)

[0076] and

R ₂ >>R _(p)

[0077] where R_(p) is the resistance through the pixel electrode and theelectro-optic medium to the electrode on the opposed side of the medium.

[0078] For amorphous silicon, the resistivity (undoped) is approximately10⁸ ohm-cm. A typical semiconductor thickness is about 500 angstroms.This information and pixel dimensions can be used to calculate therelevant resistances.

[0079] The Second Design Criterion

[0080] The minimum spacing of a pixel electrode 34 to a data line 30,L_(ms), can be derived from a consideration of the effect of the leakageon the pixel voltage. In order to avoid undesirable voltage shifts onthe pixel, the following condition must be met:

I _(leak) T _(f) ≦C _(p) ΔV _(p)

[0081] where I_(leak) is the leakage current from the data line to thepixel electrode through the unpatterned semiconductor layer, T_(f) isthe frame time, and C_(p) is the total capacitance of the pixel. ΔV_(p)is the maximum tolerance for leakage-induced voltage shifts on the pixelelectrode. This value depends on how voltage shifts affect the opticalstate of the pixel and the tolerance defined by the display parameters.

[0082] I_(leak), at the minimum spacing, can be expressed by:

I_(leak) =σwh(V _(p) −V _(d))/L_(ms)

[0083] where σ is the conductivity of the semiconductor material, w isthe width of the leakage path, h is the thickness of the underlyingsemiconductor material, and V_(d) is the voltage of the data line.

[0084] Combining the above two equations gives the following relationthat defines a minimum spacing L_(ms):

L _(ms) ≧σwh(V _(p) −V _(d))T _(f) /C _(pix) ΔV _(p).

[0085] The above discussion applies to embodiments with a single leakagesource. If there are multiple leakage sources, I_(leak) will includeleakage currents from each leakage source and the minimum spacing L_(ms)for each leakage path must be derived accordingly.

[0086] A preferred embodiment of a thin-film transistor for use in anencapsulated electrophoretic display is shown in FIG. 5a. Referring toFIG. 5a, this preferred embodiment includes data lines 30′, 32′, aselection line 36′, a pixel electrode 34′, and a capacitor top electrode92′. Various physical dimensions are indicated, in microns.

[0087] The embodiment of FIG. 5a is illustrated in cross section in FIG.5b, though not to scale. Referring to FIG. 5b, the embodiment includesbottom gate electrode 53′ and bottom capacitor electrode 55′, a siliconnitride (“SiN”) dielectric layer 54′, an amorphous silicon layer 56′,amorphous silicon contacts 58′ drain and pixel electrodes 59′ , andcapacitor top electrode 92′. Other embodiments may employ differentmaterials, for example, other dielectric materials such as silicondioxide.

[0088] To illustrate the operating characteristics of the embodiment ofFIGS. 5a and 5 b, samples were prepared through either a two-maskprocess, as preferred, or a three-mask process, for comparison. In thetwo-mask process, the amorphous silicon layer 56′ was not patternedwhile in the three mask process the amorphous silicon layer 56′ waspatterned. The physical and experimentally measured electricalcharacteristics for these two samples are given in the table below.On/Off Threshold Max. Drain Min. Drain Storage Sample WL Ratio MobilityVoltage G_(m) Current Current capacitance Patterned 200/20   1 × 10⁸ .55cm²/Vs 5.0 V 18.9 nA/V² 10 μA 0.1 pA 19.1 pF Unpatterned 160/20 3.3 ×10⁵ .43 cm²/Vs 5.0 V 23.4 nA/V² 20 μA  60 pA 18.4 pF

[0089] The leakage current and On/Off ratio for the unpatterned sample,as expected, are poorer than for the patterned sample. The unpatternedsample, however, is both suitable and preferable for many displayapplications, as discussed above. Referring to FIG. 6, the drain currentversus gate voltage characteristics of the two-mask sample are shown.The drain current can be caused to vary by over five orders of magnitudeby changing the gate voltage from zero to 30 volts. This large rangemakes this transistor suitable for many display applications.

[0090] Further alternative embodiments of a thin-film transistor arrayare now given. Referring to FIG. 7, an array of bottom gate transistors50 include a substrate 52, a patterned gate electrode 53 for eachtransistor provided adjacent the substrate 52, a dielectric layer 54provided adjacent the gate electrodes 53 and the substrate 52, anamorphous silicon layer 56 provided adjacent the dielectric layer 54, aplurality of patterned n⁺ doped amorphous silicon contact layers 58provided adjacent the amorphous silicon doped layer 56, and patternedsource, drain or pixel electrodes 59 provided adjacent the patterned n⁺doped amorphous silicon contacts layers 58. Each patterned n⁺ dopedamorphous silicon contact layers 58 is provided between the amorphoussilicon layer 56 and a patterned electrode 59 to provide betterelectrical contact. The contacts layers 58 at the metal-semiconductorinterface ensure ohmic behavior. The contacts 58 can be deposited by theaddition of PH₃ to SiH₄ in the gas phase. The contacts 58 can also beachieved by direct ion implantation of n-type dopants in selected areasof the intrinsic amorphous silicon layer 56 followed by high temperatureannealing as an alternative to the additional n⁺ amorphous silicondeposition step. The contacts 58, however, are not essential to producea sufficiently functioning transistor.

[0091] Referring to FIG. 8, an array of top gate transistors 60 includea substrate 62, patterned source, drain, and/or pixel electrodes 64 foreach transistor provided adjacent the substrate 62, a patterned n⁺amorphous silicon contact 66 provided adjacent each electrode 64, anamorphous silicon layer 68 provided adjacent the contacts 66 and thesubstrate 62, a dielectric layer 70 provided adjacent to the boron dopedamorphous silicon layer 68, and a gate electrode 72 for each transistorprovided adjacent to the dielectric layer 70.

[0092] Referring to FIG. 9, an array of bottom gate transistors 80 issubstantially similar to the transistors 50 of FIG. 7. The transistors80 of FIG. 9 include a passivation layer 82 provided above the exposedregions of the amorphous silicon layer 56. The passivation layer 82 canbe deposited after the patterning of the electrodes 59. For example, thepassivation layer 82 can consist of silicon nitride. In one embodiment,a light blocking layer is incorporated into the array of transistors toshield any exposed silicon layer 56. The light blocking layer can beeither light absorbing or reflective.

[0093] Referring to FIG. 10, an array of bottom gate transistors 90 issubstantially similar to the array of transistors 80 of FIG. 9. Thearray of transistors 90 further incorporates a substrate capacitor 292.The substrate capacitor 292 can be formed by extending the pixelelectrode 94 over the preceding gate line 53 b. The capacitance isdirectly proportional to the area of overlap.

[0094] In one alternative, inexpensive displays can be constructed byminimizing the number of patterning steps. Such a display can takedifferent forms, including but not limited to: large area displays,displays with low-to-moderate pixel density, or microencapsulatedelectrophoretic display devices. In the preferred embodiment thesemiconductor layer 18, 56, or 68 is unpatterned. Alternatively, thedielectric layer 16, 54, or 70 is unpatterned.

[0095] An electronic display can incorporate an array of transistors asdescribed above. Referring to FIG. 11, an electronic display may includea transparent over-layer 101 supporting an electrode 102, a displaymedium 106 provided next to the electrode 102, a plurality of pixelelectrodes 104 provided next to the display medium 106, and a pluralityof discrete electronic devices (e.g., transistors) provided next to andin electrical communication with the pixel electrodes 104 supported by asubstrate 110 provided next to and in electrical communication with thediscrete electronic devices. The discrete electronic devices, in thisembodiment, are transistors. The gate electrodes 112, the gatedielectric layer 100, the semiconductor layer 118 and the sourceelectrodes 120 of the transistors are shown in this cross-section.

[0096] The over-layer 101 can be made of a transparent material. Theover-layer 101 can also be a flexible substrate. For example, theover-layer 101 can consist of polyester. The electrode 102 can be acommon electrode. Alternatively, the electrode 102 can be a plurality ofrow electrodes. The electrode 102 can consist of a transparentconductive material. For example, an indium tin oxide (ITO), polyanilineor polythiophene coating can be provided on an inner surface of theover-layer 101.

[0097] The display medium 106 can include a plurality of microcapsules124 dispersed in a binder 126 (not shown in drawing). Each microcapsule124 can include an electro-optical material. An electro-optical materialrefers to a material which displays an optical property in response toan electrical signal. Electro-optical material, for example, can beelectrophoretic particles or liquid crystals dispersed in a solvent. Anelectro-optical material can also be bichromal spheres dispersed in asolvent. Details of electro-optical materials within the microcapsules124 will be discussed below. An important property of theelectro-optical material within the microcapsules 124 is that thematerial is capable of displaying one visible state upon application ofan electric field and a different visual state upon application of adifferent electric field.

[0098] In one embodiment, the display medium 106 comprises aparticle-based display medium. In one detailed embodiment, theparticle-based display medium comprises an electronic ink. An electronicink is an optoelectronically active material which comprises at leasttwo phases: an electrophoretic contrast medium phase and acoating/binding phase. The electrophoretic phase comprises, in someembodiments, a single species of electrophoretic particles dispersed ina clear or dyed medium, or more than one species of electrophoreticparticles having distinct physical and electrical characteristicsdispersed in a clear or dyed medium. In some embodiments theelectrophoretic phase is encapsulated, that is, there is a capsule wallphase between the two phases. The coating/binding phase includes, in oneembodiment, a polymer matrix that surrounds the electrophoretic phase.In this embodiment, the polymer in the polymeric binder is capable ofbeing dried, crosslinked, or otherwise cured as in traditional inks, andtherefore a printing process can be used to deposit the electronic inkonto a substrate.

[0099] The optical quality of an electronic ink is quite distinct fromother electronic display materials. The most notable difference is thatthe electronic ink provides a high degree of both reflectance andcontrast because it is pigment based (as are ordinary printing inks).The light scattered from the electronic ink comes from a very thin layerof pigment close to the top of the viewing surface. In this respect itresembles an ordinary, printed image. Also, electronic ink is easilyviewed from a wide range of viewing angles in the same manner as aprinted page, and such ink approximates a Lambertian contrast curve moreclosely than any other electronic display material. Since electronic inkcan be printed, it can be included on the same surface with any otherprinted material, including traditional inks. Electronic ink can be madeoptically stable in all display configurations, that is, the ink can beset to a persistent optical state. Fabrication of a display by printingan electronic ink is particularly useful in low power applicationsbecause of this stability.

[0100] Electronic ink displays are novel in that they can be addressedby DC voltages and draw very little current. As such, the conductiveleads and electrodes used to deliver the voltage to electronic inkdisplays can be of relatively high resistivity. The ability to useresistive conductors substantially widens the number and type ofmaterials that can be used as conductors in electronic ink displays. Inparticular, the use of costly vacuum-sputtered indium tin oxide (ITO)conductors, a standard material in liquid crystal devices, is notrequired.

[0101] Aside from cost savings, the replacement of ITO with othermaterials can provide benefits in appearance, processing capabilities(printed conductors), flexibility, and durability. Additionally, theprinted electrodes are in contact only with a solid binder, not with afluid layer (like liquid crystals). This means that some conductivematerials, which would otherwise dissolve or be degraded by contact withliquid crystals, can be used in an electronic ink application. Theseinclude opaque metallic inks for the rear electrode (e.g., silver andgraphite inks), as well as conductive transparent inks for eithersubstrate.

[0102] These conductive coatings include conducting or semiconductingcolloids, examples of which are indium tin oxide and antimony-doped tinoxide. Organic conductors (polymeric conductors and molecular organicconductors) also may be used. Polymers include, but are not limited to,polyaniline and derivatives, polythiophene and derivatives,poly(3,4-ethylenedioxythiophene) (PEDOT) and derivatives, polypyrroleand derivatives, and polyphenylenevinylene (PPV) and derivatives.Organic molecular conductors include, but are not limited to,derivatives of naphthalene, phthalocyanine, and pentacene. Polymerlayers can be made thinner and more transparent than with traditionaldisplays because conductivity requirements are not as stringent.

[0103] The pixel electrodes 104 can be bonded to the display medium 106through a binder. The binder, for example, can be a pressure sensitiveadhesive. The pixel electrodes 104 can be made from any conductivematerial. The pixel electrodes 104 can be transparent or opaque. Forexample, the pixel electrodes 104 can be made from aluminum, chrome,solder paste, copper, copper-clad polyimide, graphite inks, silver inksand other metal containing conductive inks. The pixel electrodes 104 canbe formed on a substrate 110 and subsequently bonded to the displaymedium 106.

[0104] The discrete electronic devices can be non-linear devices such astransistor for addressing the pixels of the display. Alternatively, thenon-linear devices can be diodes.

[0105] The electrodes 112, 120 can be made of any conductive material,either transparent or opaque. The conductive material can be printed,coated, or vacuum sputtered. For example, the electrodes 102, 112, 120can also be made using transparent materials such as indium tin oxideand conductive polymers such as polyaniline or polythiophenes.Alternatively, the electrodes 102, 112, 120 can be made of opaquematerials such as aluminum, chrome, solder paste, copper, copper-cladpolyimide, graphite inks, silver inks and other metal-containingconductive inks.

[0106] The architecture of the electronic display shown in FIG. 11 isexemplary only and other architectures for an electronic display mayalso be used in accordance with this invention.

[0107] Very Thin a-Si Active Layer

[0108] To reduce leakage currents, as noted in the description of FIG.1, an a-Si active layer is traditionally patterned to leave islands ofsemiconductor material. The structures shown in FIG. 1 typically requirethree lithography steps and four etching steps. In contrast, someembodiments of the invention, as described above in reference to FIG.5b, employ two masks in a simplified fabrication process. As describedin the following, another embodiment of the invention provides furtherfabrication improvements through use of very thin a-Si for the activelayer.

[0109] Referring to FIG. 12, one embodiment that employs a very thina-Si layer includes a gate electrode 53 a, a bottom capacitor electrode55 a, a SiN dielectric layer 54 a, an a-Si layer 56 a, drain and pixelelectrodes 59 a, and a capacitor top electrode 92 a. This embodiment,may be fabricated with a two-mask process, and without use of highlydoped a-Si to assist formation of ohmic contacts. The a-Si layer 56 amay be formed with no further treatment after deposition, such as achemical treatment to vary electrical properties. The a-Si layer 56 apreferably extends continuously from a transistor to neighboringtransistors that reside both in rows and columns in an array oftransistors.

[0110] A bottom gate with top pixel electrode structure is advantageousfor electro-optic display applications. Such a structure positions thepixel electrodes closely to the electro-optic display medium. Thus,drive voltage and energy consumption may be reduce. Moreover, leakagecurrent may be reduced.

[0111] In preferred embodiments, the a-Si layer 56 a has a thickness ofapproximately 40 nm or less. The use of a very thin a-Si layer as anactive layer in a TFT obviates the requirement of heavily doped n⁺ a-Silying between the a-Si layer 56 a and the electrodes 59 a. SeeThomasson, et al., IEEE Elec. Dev. Lett., Vol. 18, no. 3, 1717 (1997).For example, by employing intrinsic a-Si of 10 nm thickness, gateinduced carrier concentration substantially reduces the metal to channelSchottky barrier. Hence, carriers may tunnel from the metal source anddrain contacts to the channel, without reducing the TFT current andsubstantially affecting performance.

[0112] Elimination of an n⁺ a-Si layer at the metal to active layerinterface reduces the number and difficulty of process steps by, forexample, eliminating deposition and etching of n⁺ a-Si. This may alsopermit use of a very thin a-Si active layer due to elimination of theneed to overetch the n⁺ a-Si layer.

[0113] Use of a very thin a-Si layer as the active layer may providefurther advantages. If left unpatterned, an active layer of, forexample, 10 nm thick a-Si may reduce leakage currents due to increasedlateral resistance relative to that of a thicker, continuous activelayer. Hence, as discussed above, device dimensions may be reduced whilestill achieving acceptable leakage current levels. Thus, use of verythin a-Si as an active layer may permit dense packing of electroniccomponents while still employing a simple two-mask fabrication process.

[0114] The embodiment illustrated in FIG. 12 may be fabricated asfollows. A first metal layer is deposited and patterned to form the gateelectrode 53 a and the capacitor's bottom electrode 55 a. The SiNdielectric layer 54 a, the a-Si layer 56 a, and a second metal layer arethen deposited. The drain and pixel electrodes 59 a are formed from thesecond metal layer by, for example, wet etching.

[0115] Referring to FIGS. 13-15, electrical measurements were obtainedfrom sample TFT arrays having the structure of the embodimentillustrated in FIG. 12. FIG. 13 shows the drain current versus gatevoltage of a TFT in an array having a shared 10 nm thick a-Si layer. Thethreshold voltage is approximately 13 volts, which is somewhat greaterthan the threshold voltage of 3 to 4 volts for a typical TFT having aconventional structure. The mobility of the TFT is 0.15 cM²/Vs. Thedrain current on/off ratio is greater than 2×10⁵.

[0116]FIG. 14 shows the drain current versus drain voltage for a TFT inthe same sample array used to obtain the data presented in FIG. 13. Thecontact resistance between the source and drain electrodes and theintrinsic a-Si layer partially limits the drain current at low drainvoltage in this sample TFT. The on/off ratio, however, is good, and themobility and on-current are sufficient to drive, for example, an activematrix display pixel.

[0117]FIG. 15 shows a transient voltage switching-and-holding plot of apixel electrode in a sample 40 dpi display fabricated with TFTs similarto those used to obtain the data presented in FIG. 13 and FIG. 14. Thepixel electrode has a dynamic range of 0 to 15 volts when the voltagerange of the gate voltage and the drain voltage are set to 30 volts. Thevoltage holding range of the sample pixel is approximately 90 w. Themeasured dynamic range and voltage holding ratio are sufficient todrive, for example, an electrophoretic medium display.

[0118] Various embodiments of the invention have numerous advantagesover the prior art. TFT arrays may be fabricated at low cost.Fabrication may utilize only two patterning steps. No patterning of asemiconductor active layer is required; this may, for example, eliminatea photolithographic step and a dry etching step. A heavily dopedsemiconductor layer may be eliminated at the metal to semiconductoractive layer interface; this may, for example, eliminate a dry etchingstep.

[0119] Elimination of n⁺ a-Si from fabrication may eliminate associatedcosts that arise from the requirement of a deposition chamber, as wellas hazards entailed by use of highly toxic and flammable PH₃ gas.Related elimination of a dry etch step permits use of all-wetfabrication, further reducing fabrication costs.

[0120] The above features of the invention further permit increasedfabrication throughput. Use of a thinner semiconductor active layerreduces semiconductor deposition time. Elimination of a heavily dopedsemiconductor layer, and elimination of patterning of the semiconductoractive layer, further increase fabrication throughput. In someembodiments, a SiN layer, an a-Si layer and a metal 2 layer aredeposited in the same deposition system, again improving manufacturingthroughput.

[0121] The invention may provide improved fabrication yield, due tosimplified processing. Moreover, some embodiments may utilize aroll-to-roll substrate fabrication process. Continuous deposition of asemiconductor stack and metal 2 without a break in vacuum, as well as anall-wet etching process, are compatible with roll-to-roll processing.

[0122] Materials for Use in Electrophoretic Displays Useful materialsfor constructing encapsulated electrophoretic displays are discussed indetail below. Many of these materials will be known to those skilled inthe art of constructing conventional electrophoretic displays, or thoseskilled in the art of microencapsulation. The combination of thesematerials and processes, along with the other necessary components foundin an encapsulated electrophoretic display, comprise the inventiondescribed herein.

[0123] A. Particles

[0124] There is much flexibility in the choice of particles for use inelectrophoretic displays, as described above. For purposes of thisinvention, a particle is any component that is charged or capable ofacquiring a charge (i.e., has or is capable of acquiring electrophoreticmobility), and, in some cases, this mobility may be zero or close tozero (i.e., the particles will not move). The particles may be neatpigments, dyed (laked) pigments or pigment/polymer composites, or anyother component that is charged or capable of acquiring a charge.Typical considerations for the electrophoretic particle are its opticalproperties, electrical properties, and surface chemistry. The particlesmay be organic or inorganic compounds, and they may either absorb lightor scatter light. The particles for use in the invention may furtherinclude scattering pigments, absorbing pigments and luminescentparticles. The particles may be retroreflective, such as corner cubes,or they may be electroluminescent, such as zinc sulfide particles, whichemit light when excited by an AC field, or they may be photoluminescent.Finally, the particles may be surface treated so as to improve chargingor interaction with a charging agent, or to improve dispersibility.

[0125] A preferred particle for use in electrophoretic displays of theinvention is Titania. The titania particles may be coated with a metaloxide, such as aluminum oxide or silicon oxide, for example. The titaniaparticles may have one, two, or more layers of metal-oxide coating. Forexample, a titania particle for use in electrophoretic displays of theinvention may have a coating of aluminum oxide and a coating of siliconoxide. The coatings may be added to the particle in any order.

[0126] The electrophoretic particle is usually a pigment, a polymer, alaked pigment, or some combination of the above. A neat pigment can beany pigment, and, usually for a light colored particle, pigments suchas, for example, rutile (titania), anatase (titania), barium sulfate,kaolin, or zinc oxide are useful. Some typical particles have highrefractive indices, high scattering coefficients, and low absorptioncoefficients. Other particles are absorptive, such as carbon black orcolored pigments used in paints and inks. The pigment should also beinsoluble in the suspending fluid. Yellow pigments such as diarylideyellow, hansa yellow, and benzidin yellow have also found use in similardisplays. Any other reflective material can be employed for a lightcolored particle, including non-pigment materials, such as metallicparticles.

[0127] Useful neat pigments include, but are not limited to, PbCrO₄,Cyan blue GT 55-3295 (American Cyanamid Company, Wayne, N.J.), CibacronBlack BG (Ciba Company, Inc., Newport, Del.), Cibacron Turquoise Blue G(Ciba), Cibalon Black BGL (Ciba), Orasol Black BRG (Ciba), Orasol BlackRBL (Ciba), Acetamine Blac, CBS (E. I. du Pont de Nemours and Company,Inc., Wilmington, Del.), Crocein Scarlet N Ex (du Pont) (27290), FiberBlack VF (DuPont) (30235), Luxol Fast Black L (DuPont) (Solv. Black 17),Nirosine Base No. 424 (DuPont) (50415 B), Oil Black BG (DuPont) (Solv.Black 16), Rotalin Black RM (DuPont), Sevron Brilliant Red 3 B (DuPont);Basic Black DSC (Dye Specialties, Inc.), Hectolene Black (DyeSpecialties, Inc.), Azosol Brilliant Blue B (GAF, Dyestuff and ChemicalDivision, Wayne, N.J.) (Solv. Blue 9), Azosol Brilliant Green BA (GAF)(Solv. Green 2), Azosol Fast Brilliant Red B (GAF), Azosol Fast OrangeRA Conc. (GAF) (Solv. Orange 20), Azosol Fast Yellow GRA Conc. (GAF)(13900 A), Basic Black KMPA (GAF), Benzofix Black CW-CF (GAF) (35435),Cellitazol BNFV Ex Soluble CF (GAF) (Disp. Black 9), Celliton Fast BlueAF Ex Conc (GAF) (Disp. Blue 9), Cyper Black IA (GAF) (Basic Blk. 3),Diamine Black CAP Ex Conc (GAF) (30235), Diamond Black EAN Hi Con. CF(GAF) (15710), Diamond Black PBBA Ex (GAF) (16505); Direct Deep Black EAEx CF (GAF) (30235), Hansa Yellow G (GAF) (11680); Indanthrene Black BBKPowd. (GAF) (59850), Indocarbon CLGS Conc. CF (GAF) (53295), KatigenDeep Black NND Hi Conc. CF (GAF) (15711), Rapidogen Black 3 G (GAF)(Azoic Blk. 4); Sulphone Cyanine Black BA-CF (GAF) (26370), ZambeziBlack VD Ex Conc. (GAF) (30015); Rubanox Red CP-1495 (TheSherwin-Williams Company, Cleveland, Ohio) (15630); Raven 11 (ColumbianCarbon Company, Atlanta, Ga.), (carbon black aggregates with a particlesize of about 25 μm), Statex B-12 (Columbian Carbon Co.) (a furnaceblack of 33 μm average particle size), and chrome green.

[0128] Particles may also include laked, or dyed, pigments. Lakedpigments are particles that have a dye precipitated on them or which arestained. Lakes are metal salts of readily soluble anionic dyes. Theseare dyes of azo, triphenylmethane or anthraquinone structure containingone or more sulphonic or carboxylic acid groupings. They are usuallyprecipitated by a calcium, barium or aluminum salt onto a substrate.Typical examples are peacock blue lake (CI Pigment Blue 24) and Persianorange (lake of CI Acid Orange 7), Black M Toner (GAF) (a mixture ofcarbon black and black dye precipitated on a lake).

[0129] A dark particle of the dyed type may be constructed from anylight absorbing material, such as carbon black, or inorganic blackmaterials. The dark material may also be selectively absorbing. Forexample, a dark green pigment may be used. Black particles may also beformed by staining latices with metal oxides, such latex copolymersconsisting of any of butadiene, styrene, isoprene, methacrylic acid,methyl methacrylate, acrylonitrile, vinyl chloride, acrylic acid, sodiumstyrene sulfonate, vinyl acetate, chlorostyrene,dimethylaminopropylmethacrylamide, isocyanoethyl methacrylate andN-(isobutoxymethacrylamide), and optionally including conjugated dienecompounds such as diacrylate, triacrylate, dimethylacrylate andtrimethacrylate. Black particles may also be formed by a dispersionpolymerization technique.

[0130] In the systems containing pigments and polymers, the pigments andpolymers may form multiple domains within the electrophoretic particle,or be aggregates of smaller pigment/polymer combined particles.Alternatively, a central pigment core may be surrounded by a polymershell. The pigment, polymer, or both can contain a dye. The opticalpurpose of the particle may be to scatter light, absorb light, or both.Useful sizes may range from 1 nm up to about 100 μm, as long as theparticles are smaller than the bounding capsule. In a preferredembodiment, the density of the electrophoretic particle may besubstantially matched to that of the suspending (i.e., electrophoretic)fluid. As defined herein, a suspending fluid has a density that is“substantially matched” to the density of the particle if the differencein their respective densities is between about zero and about two g/ml.This difference is preferably between about zero and about 0.5 g/ml.

[0131] Useful polymers for the particles include, but are not limitedto: polystyrene, polyethylene, polypropylene, phenolic resins, Du PontElvax resins (ethylene-vinyl acetate copolymers), polyesters,polyacrylates, polymethacrylates, ethylene acrylic acid or methacrylicacid copolymers (Nucrel Resins—DuPont, Primacor Resins—Dow Chemical),acrylic copolymers and terpolymers (Elvacite Resins, DuPont) and PMMA.Useful materials for homopolymer/pigment phase separation in high shearmelt include, but are not limited to, polyethylene, polypropylene,polymethylmethacrylate, polyisobutylmethacrylate, polystyrene,polybutadiene, polyisoprene, polyisobutylene, polylauryl methacrylate,polystearyl methacrylate, polyisobornyl methacrylate, poly-t-butylmethacrylate, polyethyl methacrylate, polymethyl acrylate, polyethylacrylate, polyacrylonitrile, and copolymers of two or more of thesematerials. Some useful pigment/polymer complexes that are commerciallyavailable include, but are not limited to, Process Magenta PM 1776(Magruder Color Company, Inc., Elizabeth, N.J.), Methyl Violet PMAVM6223 (Magruder Color Company, Inc., Elizabeth, N.J.), and Naphthol FGRRF6257 (Magruder Color Company, Inc., Elizabeth, N.J.).

[0132] The pigment-polymer composite may be formed by a physicalprocess, (e.g., attrition or ball milling), a chemical process (e.g.,microencapsulation or dispersion polymerization), or any other processknown in the art of particle production. From the following non-limitingexamples, it may be seen that the processes and materials for both thefabrication of particles and the charging thereof are generally derivedfrom the art of liquid toner, or liquid immersion development. Thus anyof the known processes from liquid development are particularly, but notexclusively, relevant.

[0133] New and useful electrophoretic particles may still be discovered,but a number of particles already known to those skilled in the art ofelectrophoretic displays and liquid toners can also prove useful. Ingeneral, the polymer requirements for liquid toners and encapsulatedelectrophoretic inks are similar, in that the pigment or dye must beeasily incorporated therein, either by a physical, chemical, orphysicochemical process, may aid in the colloidal stability, and maycontain charging sites or may be able to incorporate materials whichcontain charging sites. One general requirement from the liquid tonerindustry that is not shared by encapsulated electrophoretic inks is thatthe toner must be capable of “fixing” the image, i.e., heat fusingtogether to create a uniform film after the deposition of the tonerparticles.

[0134] Typical manufacturing techniques for particles are drawn from theliquid toner and other arts and include ball milling, attrition, jetmilling, etc. The process will be illustrated for the case of apigmented polymeric particle. In such a case the pigment is compoundedin the polymer, usually in some kind of high shear mechanism such as ascrew extruder. The composite material is then (wet or dry) ground to astarting size of around 10 μm. It is then dispersed in a carrier liquid,for example ISOPAR® (Exxon, Houston, Tex.), optionally with some chargecontrol agent(s), and milled under high shear for several hours down toa final particle size and/or size distribution.

[0135] Another manufacturing technique for particles drawn from theliquid toner field is to add the polymer, pigment, and suspending fluidto a media mill. The mill is started and simultaneously heated totemperature at which the polymer swells substantially with the solvent.This temperature is typically near 100° C. In this state, the pigment iseasily encapsulated into the swollen polymer. After a suitable time,typically a few hours, the mill is gradually cooled back to ambienttemperature while stirring. The milling may be continued for some timeto achieve a small enough particle size, typically a few microns indiameter. The charging agents may be added at this time. Optionally,more suspending fluid may be added.

[0136] Chemical processes such as dispersion polymerization, mini- ormicro-emulsion polymerization, suspension polymerization precipitation,phase separation, solvent evaporation, in situ polymerization, seededemulsion polymerization, or any process which falls under the generalcategory of microencapsulation may be used. A typical process of thistype is a phase separation process wherein a dissolved polymericmaterial is precipitated out of solution onto a dispersed pigmentsurface through solvent dilution, evaporation, or a thermal change.Other processes include chemical means for staining polymeric latices,for example with metal oxides or dyes.

[0137] B. Suspending Fluid

[0138] The suspending fluid containing the particles can be chosen basedon properties such as density, refractive index, and solubility. Apreferred suspending fluid has a low dielectric constant (about 2), highvolume resistivity (about 10^ 15 ohm-cm), low viscosity (less than 5cst), low toxicity and environmental impact, low water solubility (lessthan 10 ppm), high specific gravity (greater than 1.5), a high boilingpoint (greater than 90° C.), and a low refractive index (less than 1.2).

[0139] The choice of suspending fluid may be based on concerns ofchemical inertness, density matching to the electrophoretic particle, orchemical compatibility with both the electrophoretic particle andbounding capsule. The viscosity of the fluid should be low when you wantthe particles to move. The refractive index of the suspending fluid mayalso be substantially matched to that of the particles. As used herein,the refractive index of a suspending fluid “is substantially matched” tothat of a particle if the difference between their respective refractiveindices is between about zero and about 0.3, and is preferably betweenabout 0.05 and about 0.2.

[0140] Additionally, the fluid may be chosen to be a poor solvent forsome polymers, which is advantageous for use in the fabrication ofmicroparticles because it increases the range of polymeric materialsuseful in fabricating particles of polymers and pigments. Organicsolvents, such as halogenated organic solvents, saturated linear orbranched hydrocarbons, silicone oils, and low molecular weighthalogen-containing polymers are some useful suspending fluids. Thesuspending fluid may comprise a single fluid. The fluid will, however,often be a blend of more than one fluid in order to tune its chemicaland physical properties. Furthermore, the fluid may contain surfacemodifiers to modify the surface energy or charge of the electrophoreticparticle or bounding capsule. Reactants or solvents for themicroencapsulation process (oil soluble monomers, for example) can alsobe contained in the suspending fluid. Charge control agents can also beadded to the suspending fluid.

[0141] Useful organic solvents include, but are not limited to,epoxides, such as, for example, decane epoxide and dodecane epoxide;vinyl ethers, such as, for example, cyclohexyl vinyl ether and Decave®(International Flavors & Fragrances, Inc., New York, N.Y.); and aromatichydrocarbons, such as, for example, toluene and naphthalene. Usefulhalogenated organic solvents include, but are not limited to,tetrafluorodibromoethylene, tetrachloroethylene,trifluorochloroethylene, 1,2,4-trichlorobenzene, carbon tetrachloride.These materials have high densities. Useful hydrocarbons include, butare not limited to, dodecane, tetradecane, the aliphatic hydrocarbons inthe Isopar® series (Exxon, Houston, Tex.), Norpar® (series of normalparaffinic liquids), Shell-Sol® (Shell, Houston, Tex.), and Sol-Trol®(Shell), naphtha, and other petroleum solvents. These materials usuallyhave low densities. Useful examples of silicone oils include, but arenot limited to, octamethyl cyclosiloxane and higher molecular weightcyclic siloxanes, poly (methyl phenyl siloxane), hexamethyldisiloxane,and polydimethylsiloxane. These materials usually have low densities.Useful low molecular weight halogen-containing polymers include, but arenot limited to, poly(chlorotrifluoroethylene) polymer (Halogenatedhydrocarbon Inc., River Edge, N.J.), Galden® (a perfluorinated etherfrom Ausimont, Morristown, N.J.), or Krytox® from DuPont (Wilmington,Del.). In a preferred embodiment, the suspending fluid is apoly(chlorotrifluoroethylene) polymer. In a particularly preferredembodiment, this polymer has a degree of polymerization from about 2 toabout 10. Many of the above materials are available in a range ofviscosities, densities, and boiling points.

[0142] The fluid must be capable of being formed into small dropletsprior to a capsule being formed. Processes for forming small dropletsinclude flow-through jets, membranes, nozzles, or orifices, as well asshear-based emulsifying schemes. The formation of small drops may beassisted by electrical or sonic fields. Surfactants and polymers can beused to aid in the stabilization and emulsification of the droplets inthe case of an emulsion type encapsulation. A preferred surfactant foruse in displays of the invention is sodium dodecylsulfate.

[0143] It can be advantageous in some displays for the suspending fluidto contain an optically absorbing dye. This dye must be soluble in thefluid, but will generally be insoluble in the other components of thecapsule. There is much flexibility in the choice of dye material. Thedye can be a pure compound, or blends of dyes to achieve a particularcolor, including black. The dyes can be fluorescent, which would producea display in which the fluorescence properties depend on the position ofthe particles. The dyes can be photoactive, changing to another color orbecoming colorless upon irradiation with either visible or ultravioletlight, providing another means for obtaining an optical response. Dyescould also be polymerizable, forming a solid absorbing polymer insidethe bounding shell.

[0144] There are many dyes that can be chosen for use in encapsulatedelectrophoretic display. Properties important here include lightfastness, solubility in the suspending liquid, color, and cost. Theseare generally from the class of azo, anthraquinone, and triphenylmethanetype dyes and may be chemically modified so as to increase thesolubility in the oil phase and reduce the adsorption by the particlesurface.

[0145] A number of dyes already known to those skilled in the art ofelectrophoretic displays will prove useful. Useful azo dyes include, butare not limited to: the Oil Red dyes, and the Sudan Red and Sudan Blackseries of dyes. Useful anthraquinone dyes include, but are not limitedto: the Oil Blue dyes, and the Macrolex Blue series of dyes. Usefultriphenylmethane dyes include, but are not limited to, Michler's hydrol,Malachite Green, Crystal Violet, and Auramine O.

[0146] C. Charge Control Agents and Particle Stabilizers

[0147] Charge control agents are used to provide good electrophoreticmobility to the electrophoretic particles. Stabilizers are used toprevent agglomeration of the electrophoretic particles, as well asprevent the electrophoretic particles from irreversibly depositing ontothe capsule wall. Either component can be constructed from materialsacross a wide range of molecular weights (low molecular weight,oligomeric, or polymeric), and may be pure or a mixture. In particular,suitable charge control agents are generally adapted from the liquidtoner art. The charge control agent used to modify and/or stabilize theparticle surface charge is applied as generally known in the arts ofliquid toners, electrophoretic displays, non-aqueous paint dispersions,and engine-oil additives. In all of these arts, charging species may beadded to non-aqueous media in order to increase electrophoretic mobilityor increase electrostatic stabilization. The materials can improvesteric stabilization as well. Different theories of charging arepostulated, including selective ion adsorption, proton transfer, andcontact electrification.

[0148] An optional charge control agent or charge director may be used.These constituents typically consist of low molecular weightsurfactants, polymeric agents, or blends of one or more components andserve to stabilize or otherwise modify the sign and/or magnitude of thecharge on the electrophoretic particles.

[0149] The charging properties of the pigment itself may be accountedfor by taking into account the acidic or basic surface properties of thepigment, or the charging sites may take place on the carrier resinsurface (if present), or a combination of the two. Additional pigmentproperties which may be relevant are the particle size distribution, thechemical composition, and the lightfastness. The charge control agentused to modify and/or stabilize the particle surface charge is appliedas generally known in the arts of liquid toners, electrophoreticdisplays, non-aqueous paint dispersions, and engine-oil additives. Inall of these arts, charging species may be added to non-aqueous media inorder to increase electrophoretic mobility or increase electrostaticstabilization. The materials can improve steric stabilization as well.Different theories of charging are postulated, including selective ionadsorption, proton transfer, and contact electrification.

[0150] Charge adjuvants may also be added. These materials increase theeffectiveness of the charge control agents or charge directors. Thecharge adjuvant may be a polyhydroxy compound or an aminoalcoholcompound, which are preferably soluble in the suspending fluid in anamount of at least 2% by weight. Examples of polyhydroxy compounds whichcontain at least two hydroxyl groups include, but are not limited to,ethylene glycol, 2,4,7,9-tetramethyl-decyne-4,7-diol, poly(propyleneglycol), pentaethylene glycol, tripropylene glycol, triethylene glycol,glycerol, pentaerythritol, glycerol tris(12-hydroxystearate), propyleneglycerol monohydroxystearate, and ethylene glycol monohydroxystrearate.Examples of aminoalcohol compounds which contain at least one alcoholfunction and one amine function in the same molecule include, but arenot limited to, triisopropanolamine, triethanolamine, ethanolamine,3-amino-1-propanol, o-aminophenol, 5-amino-1-pentanol, andtetrakis(2-hydroxyethyl)ethylene-diamine. The charge adjuvant ispreferably present in the suspending fluid in an amount of about 1 toabout 100 mg/g of the particle mass, and more preferably about 50 toabout 200 mg/g.

[0151] The surface of the particle may also be chemically modified toaid dispersion, to improve surface charge, and to improve the stabilityof the dispersion, for example. Surface modifiers include organicsiloxanes, organohalogen silanes and other functional silane couplingagents (Dow Corning® Z-6070, Z-6124, and 3 additive, Midland, Mich.);organic titanates and zirconates (Tyzor® TOT, TBT, and TE Series,DuPont, Wilmington, Del.); hydrophobing agents, such as long chain (C12to C50) alkyl and alkyl benzene sulphonic acids, fatty amines ordiamines and their salts or quaternary derivatives; and amphipathicpolymers which can be covalently bonded to the particle surface.

[0152] In general, it is believed that charging results as an acid-basereaction between some moiety present in the continuous phase and theparticle surface. Thus useful materials are those which are capable ofparticipating in such a reaction, or any other charging reaction asknown in the art.

[0153] Different non-limiting classes of charge control agents which areuseful include organic sulfates or sulfonates, metal soaps, block orcomb copolymers, organic amides, organic zwitterions, and organicphosphates and phosphonates. Useful organic sulfates and sulfonatesinclude, but are not limited to, sodium bis(2-ethyl hexyl)sulfosuccinate, calcium dodecyl benzene sulfonate, calcium petroleumsulfonate, neutral or basic barium dinonylnaphthalene sulfonate, neutralor basic calcium dinonylnaphthalene sulfonate, dodecylbenzenesulfonicacid sodium salt, and ammonium lauryl sulphate. Useful metal soapsinclude, but are not limited to, basic or neutral barium petronate,calcium petronate, Co—, Ca—, Cu—, Mn—, Ni—, Zn—, and Fe— salts ofnaphthenic acid, Ba—, Al—, Zn—, Cu—, Pb—, and Fe— salts of stearic acid,divalent and trivalent metal carboxylates, such as aluminum tristearate,aluminum octoanate, lithium heptanoate, iron stearate, iron distearate,barium stearate, chromium stearate, magnesium octanoate, calciumstearate, iron naphthenate, and zinc naphthenate, Mn— and Zn—heptanoate, and Ba—, Al—, Co—, Mn—, and Zn— octanoate. Useful block orcomb copolymers include, but are not limited to, AB diblock copolymersof (A) polymers of 2-(N,N)-dimethylaminoethyl methacrylate quaternizedwith methyl-p-toluenesulfonate and (B) poly-2-ethylhexyl methacrylate,and comb graft copolymers with oil soluble tails of poly(12-hydroxystearic acid) and having a molecular weight of about 1800,pendant on an oil-soluble anchor group of poly (methylmethacrylate-methacrylic acid). Useful organic amides include, but arenot limited to, polyisobutylene succinimides such as OLOA 1200 and 3700,and N-vinyl pyrrolidone polymers. Useful organic zwitterions include,but are not limited to, lecithin. Useful organic phosphates andphosphonates include, but are not limited to, the sodium salts ofphosphated mono- and di-glycerides with saturated and unsaturated acidsubstituents.

[0154] Particle dispersion stabilizers may be added to prevent particleflocculation or attachment to the capsule walls. For the typical highresistivity liquids used as suspending fluids in electrophoreticdisplays, nonaqueous surfactants may be used. These include, but are notlimited to, glycol ethers, acetylenic glycols, alkanolamides, sorbitolderivatives, alkyl amines, quaternary amines, imidazolines, dialkyloxides, and sulfosuccinates.

[0155] D. Encapsulation

[0156] There is a long and rich history to encapsulation, with numerousprocesses and polymers having proven useful in creating capsules.

[0157] Encapsulation of the internal phase may be accomplished in anumber of different ways. Numerous suitable procedures formicroencapsulation are detailed in both Microencapsulation, Processesand Applications, (I. E. Vandegaer, ed.), Plenum Press, New York, N.Y.(1974) and Gutcho, Microcapsules and Mircroencapsulation Techniques,Nuyes Data Corp., Park Ridge, N.J. (1976). The processes fall intoseveral general categories, all of which can be applied to the presentinvention: interfacial polymerization, in situ polymerization, physicalprocesses, such as coextrusion and other phase separation processes,in-liquid curing, and simple/complex coacervation.

[0158] Numerous materials and processes should prove useful informulating displays of the present invention. Useful materials forsimple coacervation processes include, but are not limited to, gelatin,polyvinyl alcohol, polyvinyl acetate, and cellulosic derivatives, suchas, for example, carboxymethylcellulose. Useful materials for complexcoacervation processes include, but are not limited to, gelatin, acacia,carageenan, carboxymethylcellulose, hydrolyzed styrene anhydridecopolymers, agar, alginate, casein, albumin, methyl vinyl etherco-maleic anhydride, and cellulose phthalate. Useful materials for phaseseparation processes include, but are not limited to, polystyrene, PMMA,polyethyl methacrylate, polybutyl methacrylate, ethyl cellulose,polyvinyl pyridine, and poly acrylonitrile. Useful materials for in situpolymerization processes include, but are not limited to,polyhydroxyamides, with aldehydes, melamine, or urea and formaldehyde;water-soluble oligomers of the condensate of melamine, or urea andformaldehyde; and vinyl monomers, such as, for example, styrene, MMA andacrylonitrile. Finally, useful materials for interfacial polymerizationprocesses include, but are not limited to, diacyl chlorides, such as,for example, sebacoyl, adipoyl, and di- or poly- amines or alcohols, andisocyanates. Useful emulsion polymerization materials may include, butare not limited to, styrene, vinyl acetate, acrylic acid, butylacrylate, t-butyl acrylate, methyl methacrylate, and butyl methacrylate.

[0159] Capsules produced may be dispersed into a curable carrier,resulting in an ink which may be printed or coated on large andarbitrarily shaped or curved surfaces using conventional printing andcoating techniques. In the context of the present invention, one skilledin the art will select an encapsulation procedure and wall materialbased on the desired capsule properties. These properties include thedistribution of capsule radii; electrical, mechanical, diffusion, andoptical properties of the capsule wall; and chemical compatibility withthe internal phase of the capsule.

[0160] The capsule wall generally has a high electrical resistivity.Although it is possible to use walls with relatively low resistivities,this may limit performance in requiring relatively higher addressingvoltages. The capsule wall should also be mechanically strong (althoughif the finished capsule powder is to be dispersed in a curable polymericbinder for coating, mechanical strength is not as critical). The capsulewall should generally not be porous. If, however, it is desired to usean encapsulation procedure that produces porous capsules, these can beovercoated in a post-processing step (i.e., a second encapsulation).Moreover, if the capsules are to be dispersed in a curable binder, thebinder will serve to close the pores. The capsule walls should beoptically clear. The wall material may, however, be chosen to match therefractive index of the internal phase of the capsule (i.e., thesuspending fluid) or a binder in which the capsules are to be dispersed.For some applications (e.g., interposition between two fixedelectrodes), monodispersed capsule radii are desirable.

[0161] An encapsulation procedure involves a polymerization between ureaand formaldehyde in an aqueous phase of an oil/water emulsion in thepresence of a negatively charged, carboxyl-substituted, linearhydrocarbon polyelectrolyte material. The resulting capsule wall is aurea/formaldehyde copolymer, which discretely encloses the internalphase. The capsule is clear, mechanically strong, and has goodresistivity properties.

[0162] The related technique of in situ polymerization utilizes anoil/water emulsion, which is formed by dispersing the electrophoreticcomposition (i.e., the dielectric liquid containing a suspension of thepigment particles) in an aqueous environment. The monomers polymerize toform a polymer with higher affinity for the internal phase than for theaqueous phase, thus condensing around the emulsified oily droplets. Inone especially useful in situ polymerization processes, urea andformaldehyde condense in the presence of poly(acrylic acid) (See, e.g.,U.S. Pat. No. 4,001,140). In other useful process, any of a variety ofcross-linking agents borne in aqueous solution is deposited aroundmicroscopic oil droplets. Such cross-linking agents include aldehydes,especially formaldehyde, glyoxal, or glutaraldehyde; alum; zirconiumsalts; and poly isocyanates. The entire disclosures of the 4,001,140 and4,273,672 patents are hereby incorporated by reference herein.

[0163] The coacervation approach also utilizes an oil/water emulsion.One or more colloids are coacervated (i.e., agglomerated) out of theaqueous phase and deposited as shells around the oily droplets throughcontrol of temperature, pH and/or relative concentrations, therebycreating the microcapsule. Materials suitable for coacervation includegelatins and gum arabic.

[0164] The interfacial polymerization approach relies on the presence ofan oil-soluble monomer in the electrophoretic composition, which onceagain is present as an emulsion in an aqueous phase. The monomers in theminute hydrophobic droplets react with a monomer introduced into theaqueous phase, polymerizing at the interface between the droplets andthe surrounding aqueous medium and forming shells around the droplets.Although the resulting walls are relatively thin and may be permeable,this process does not require the elevated temperatures characteristicof some other processes, and therefore affords greater flexibility interms of choosing the dielectric liquid.

[0165] Coating aids can be used to improve the uniformity and quality ofthe coated or printed electrophoretic ink material. Wetting agents aretypically added to adjust the interfacial tension at thecoating/substrate interface and to adjust the liquid/air surfacetension. Wetting agents include, but are not limited to, anionic andcationic surfactants, and nonionic species, such as silicone orfluoropolymer based materials. Dispersing agents may be used to modifythe interfacial tension between the capsules and binder, providingcontrol over flocculation and particle settling.

[0166] Surface tension modifiers can be added to adjust the air/inkinterfacial tension. Polysiloxanes are typically used in such anapplication to improve surface leveling while minimizing other defectswithin the coating. Surface tension modifiers include, but are notlimited to, fluorinated surfactants, such as, for example, the Zonyl®series from DuPont (Wilmington, Del.), the Fluorod® series from 3M (St.Paul, Minn.), and the fluoroakyl series from Autochem (Glen Rock, N.J.);siloxanes, such as, for example, Silwet® from Union Carbide (Danbury,Conn.); and polyethoxy and polypropoxy alcohols. Antifoams, such assilicone and silicone-free polymeric materials, may be added to enhancethe movement of air from within the ink to the surface and to facilitatethe rupture of bubbles at the coating surface. Other useful antifoamsinclude, but are not limited to, glyceryl esters, polyhydric alcohols,compounded antifoams, such as oil solutions of alkyl benzenes, naturalfats, fatty acids, and metallic soaps, and silicone antifoaming agentsmade from the combination of dimethyl siloxane polymers and silica.Stabilizers such as uv-absorbers and antioxidants may also be added toimprove the lifetime of the ink.

[0167] Other additives to control properties like coating viscosity andfoaming can also be used in the coating fluid. Stabilizers(UV-absorbers, antioxidants) and other additives which could proveuseful in practical materials.

[0168] E. Binder Material

[0169] The binder is used as a non-conducting, adhesive mediumsupporting and protecting the capsules, as well as binding the electrodematerials to the capsule dispersion. Binders are available in many formsand chemical types. Among these are water-soluble polymers, water-bornepolymers, oil-soluble polymers, thermoset and thermoplastic polymers,and radiation-cured polymers.

[0170] Among the water-soluble polymers are the various polysaccharides,the polyvinyl alcohols, N-methylpyrrolidone, N-vinylpyrrollidone, thevarious Carbowax® species (Union Carbide, Danbury, Conn.), andpoly-2-hydroxyethylacrylate.

[0171] The water-dispersed or water-borne systems are generally latexcompositions, typified by the Neorez® and Neocryl® resins (ZenecaResins, Wilmington, Mass.), Acrysol® (Rohm and Haas, Philadelphia, Pa.),Bayhydrol® (Bayer, Pittsburgh, Pa.), and the Cytec Industries (WestPaterson, N.J.) HP line. These are generally latices of polyurethanes,occasionally compounded with one or more of the acrylics, polyesters,polycarbonates or silicones, each lending the final cured resin in aspecific set of properties defined by glass transition temperature,degree of “tack,” softness, clarity, flexibility, water permeability andsolvent resistance, elongation modulus and tensile strength,thermoplastic flow, and solids level. Some water-borne systems can bemixed with reactive monomers and catalyzed to form more complex resins.Some can be further cross-linked by the use of a crosslinking reagent,such as an aziridine, for example, which reacts with carboxyl groups.

[0172] A typical application of a water-borne resin and aqueous capsulesfollows. A volume of particles is centrifuged at low speed to separateexcess water. After a given centrifugation process, for example 10minutes at 60×G, the capsules are found at the bottom of the centrifugetube, while the water portion is at the top. The water portion iscarefully removed (by decanting or pipetting). The mass of the remainingcapsules is measured, and a mass of resin is added such that the mass ofresin is between one eighth and one tenth of the weight of the capsules.This mixture is gently mixed on an oscillating mixer for approximatelyone half hour. After about one half hour, the mixture is ready to becoated onto the appropriate substrate.

[0173] The thermoset systems are exemplified by the family of epoxies.These binary systems can vary greatly in viscosity, and the reactivityof the pair determines the “pot life” of the mixture. If the pot life islong enough to allow a coating operation, capsules may be coated in anordered arrangement in a coating process prior to the resin curing andhardening.

[0174] Thermoplastic polymers, which are often polyesters, are molten athigh temperatures. A typical application of this type of product ishot-melt glue. A dispersion of heat-resistant capsules could be coatedin such a medium. The solidification process begins during cooling, andthe final hardness, clarity and flexibility are affected by thebranching and molecular weight of the polymer.

[0175] Oil or solvent-soluble polymers are often similar in compositionto the water-borne system, with the obvious exception of the wateritself. The latitude in formulation for solvent systems is enormous,limited only by solvent choices and polymer solubility. Of considerableconcern in solvent-based systems is the viability of the capsuleitself—the integrity of the capsule wall cannot be compromised in anyway by the solvent.

[0176] Radiation cure resins are generally found among the solvent-basedsystems. Capsules may be dispersed in such a medium and coated, and theresin may then be cured by a timed exposure to a threshold level of veryviolet radiation, either long or short wavelength. As in all cases ofcuring polymer resins, final properties are determined by the branchingand molecular weights of the monomers, oligomers and crosslinkers.

[0177] A number of “water-reducible” monomers and oligomers are,however, marketed. In the strictest sense, they are not water soluble,but water is an acceptable diluent at low concentrations and can bedispersed relatively easily in the mixture. Under these circumstances,water is used to reduce the viscosity (initially from thousands tohundreds of thousands centipoise). Water-based capsules, such as thosemade from a protein or polysaccharide material, for example, could bedispersed in such a medium and coated, provided the viscosity could besufficiently lowered. Curing in such systems is generally by ultravioletradiation.

[0178] Referring to FIG. 16a, an embodiment of an electrophoreticdisplay that employs a thin-film transistor array of the presentinvention is shown. FIG. 16a shows a diagrammatic cross-section of anelectrophoretic display 130 constructed using electronic ink. The binder132 includes at least one capsule 134, which is filled with a pluralityof particles 136 and a dyed suspending fluid 138. In one embodiment, theparticles 136 are titania particles. When a direct-current electricfield of the appropriate polarity is applied across the capsule 134, theparticles 136 move to the viewed surface of the display and scatterlight. When the applied electric field is reversed, the particles 136move to the rear surface of the display and the viewed surface of thedisplay then appears dark.

[0179]FIG. 16b shows a cross-section of another electrophoretic display140 constructed using electronic ink. This display comprises a first setof particles 142 and a second set of particles 144 in a capsule 141. Thefirst set of particles 142 and the second set of particles 144 havecontrasting optical properties. For example, the first set of particles142 and the second set of particles 144 can have differingelectrophoretic mobilities. In addition, the first set of particles 142and the second set of particles 144 can have contrasting colors. Forexample, the first set of particles 142 can be white, while the secondset of particles 144 can be black. The capsule 141 further includes asubstantially clear fluid. The capsule 141 has electrodes 146 and 146′disposed adjacent it. The electrodes 146, 146′ are connected to a sourceof voltage 148, which may provide an electric field to the capsule 141.In one embodiment, upon application of an electric field across theelectrodes 146, 146′, the first set of particles 142 move towardelectrode 146′, while the second set of particles 144 move towardelectrode 146. In another embodiment, upon application of an electricfield across the electrodes 146, 146′, the first set of particles 142move rapidly toward electrode 146′, while the second set of particles144 move only slowly or not at all towards electrode 146, so that thefirst set of particles packs preferentially at the microcapsule surfaceadjacent to electrode 146′.

[0180]FIG. 16c shows a diagrammatic cross-section of a suspendedparticle display 250. The suspended particle display 250 includesneedle-like particles 252 in a transparent fluid 254. The particles 252change their orientation upon application of an AC field across theelectrodes 256, 256′. When the AC field is applied, the particles 252are oriented perpendicular with respect to the display surface and thedisplay appears transparent. When the AC field is removed, the particles252 are randomly oriented and the display 250 appears opaque.

[0181] In another detailed embodiment, a display 160 can comprise aplurality of bichromal spheres, as illustrated in FIG. 16d. A bichromalsphere typically comprises a positively charged hemisphere 162 of afirst color and a negatively charged hemisphere 164 of a second color ina liquid medium 166. Upon application of an electric field across thesphere through a pair of electrodes 168, 168′, the sphere rotates anddisplays the color of one of the two hemispheres 162, 164.

[0182] In an alternative embodiment, an array of transistors withreduced cross-talk is prepared by increasing the resistivity of thesemiconductor layer. For example, where the semiconductor layer is anamorphous silicon that is slightly n-type as deposited, thesemiconductor can be lightly doped with boron or an equivalent p-typedopant to increase the resistivity of the semiconductor layer. If thesemiconductor layer is doped with too much boron, the semiconductorlayer will become p-type and the resistivity will decrease. For example,in a display application, the boron doping can be adjusted to providethe minimum required “on” current for the transistor to drive a pixel ofa display, while concurrently maintaining sufficient isolation betweenneighboring elements or signals. As discussed, the spacing betweenneighboring source and drain electrodes of the transistors and the metalsignal lines must be sufficiently large to suppress charge leakagethrough the underlying semiconductor layer in this embodiment. Thisminimum spacing can be derived via a resistance calculation if theleakage current, electrode potential, semiconductor conductivity andthickness of various materials are known.

[0183] While the invention has been particularly shown and describedwith reference to specific preferred embodiments, it should beunderstood by those skilled in the art that various changes in form anddetail may be made therein without departing from the spirit and scopeof the invention as defined by the appended claims. For example, anarray of active or passive elements can be prepared in accordance withthe present invention. The array of elements can be used in devicesother than displays.

What is claimed is:
 1. A thin-film transistor array comprising at leastfirst and second transistors, each of the first and second transistorscomprising: a shared silicon layer having a thickness less than 40 nmand extending continuously between the first and second transistors; asource electrode in direct contact with the silicon layer; a drainelectrode spaced from the source electrode and in direct contact withthe silicon layer; and a gate electrode disposed adjacent to the siliconlayer.
 2. The thin-film transistor array of claim 1 wherein the siliconlayer consists of unpatterned silicon.
 3. The thin-film transistor arrayof claim 2 wherein the silicon layer consists of amorphous silicon. 4.The thin-film transistor array of claim 1 wherein the silicon layer isundoped.
 5. The thin-film transistor array of claim 1 wherein the firsttransistor is a bottom gate transistor.
 6. The thin-film transistorarray of claim 1 wherein the first transistor is a top gate transistor.7. The thin-film transistor array of claim 1, the first transistorfurther comprising a first pixel electrode of an electronic display, thefirst pixel electrode in communication with the source electrode of thefirst transistor, and the drain electrode of the first transistor is incommunication with a first data line of the electronic display.
 8. Thethin-film transistor array of claim 7 wherein a distance between thefirst pixel electrode and the first data line is selected to provide anacceptable leakage current between the first pixel electrode and thefirst data line.
 9. The thin-film transistor array of claim 7 wherein adistance between the first transistor and the second transistor isselected to provide an acceptable leakage current between the first dataline and the second data line.
 10. The thin-film transistor array ofclaim 9 wherein at least one of the first data line, the second dataline, the first transistor and the first pixel electrode have a geometryselected to provide an acceptable leakage between the first data lineand the second data line.
 11. An electronic display comprising: adisplay medium; a first pixel electrode and a second pixel electrodeprovided adjacent to the display medium; and a first thin-filmtransistor and a second thin-film transistor in respective electricalcommunication with the first pixel electrode and the second pixelelectrode, and comprising a shared continuous amorphous silicon layerthat has a thickness less than 40 nm and provides channels for the firstthin-film transistor and the second thin-film transistor.
 12. Theelectronic display of claim 11 wherein the display medium iselectrophoretic.
 13. The electronic display of claim 12 wherein theelectrophoretic medium comprises at least one type of particle and asuspending fluid.
 14. The electronic display of claim 12 wherein theelectrophoretic medium is encapsulated.
 15. The electronic display ofclaim 11 further comprising a light blocking layer provided adjacent tothe silicon layer.
 16. The electronic display of claim 11 furthercomprising a first data line in communication with the first transistorand a second data line in communication with the second transistor,wherein a distance between the first transistor and the secondtransistor is selected to provide an acceptable leakage between thefirst data line and the second data line.
 17. The electronic display ofclaim 16 wherein a distance between the first pixel electrode and thefirst data line is selected to provide an acceptable leakage between thefirst pixel electrode and the first data line.
 18. The electronicdisplay of claim 11 wherein the first transistor comprises a gateelectrode, a source electrode and a drain electrode and the gateelectrode and one of the source electrode and the drain electrode form acapacitor.
 19. A method of manufacturing an array of thin-filmtransistors comprising at least a first transistor and a secondtransistor, the method comprising the steps of: providing a substrate;forming adjacent to the substrate an unpatterned silicon layer having athickness less than 40 nm; forming at least one patterned drainelectrode for each of the transistors, the drain electrodes in directcontact with the unpatterned silicon layer; forming at least onepatterned source electrode for each of the transistors, the sourceelectrodes in direct contact with the unpatterned silicon layer; andforming at least one gate electrode for each of the transistors, thegate electrode disposed adjacent to the unpatterned silicon layer. 20.The method of claim 19 further comprising the step of selecting aspacing between the first transistor and the second transistor toprovide an acceptable leakage current between the first transistor andthe second transistor.
 21. The method of claim 19 further comprising thestep of forming a dielectric layer adjacent to the at least one gateelectrode.
 22. The method of claim 19 wherein the step of providing asubstrate comprises unwinding the substrate from a first roll andwinding the substrate onto a second roll.
 23. The method of claim 21wherein the steps of forming the dielectric layer, forming theunpatterned silicon layer and forming the source and drain electrodesoccur at least partially during one visit of the substrate inside asingle deposition chamber.
 24. The method of claim 19 further comprisingthe steps of: providing a first pixel electrode of an electronic displayin communication with the source electrode of the first transistor; andproviding a first data line of the electronic display in communicationwith the drain electrode of the first transistor.
 25. The method ofclaim 24 further comprising the steps of: providing a second pixelelectrode of an electronic display in communication with the sourceelectrode of the second transistor; providing a second data line of theelectronic display in communication with the drain electrode of thesecond transistor; and selecting a geometry of at least one of: (i) thefirst data line; (ii) the second data line; (iii) the first transistorand (iv) the first pixel electrode, to provide an acceptable leakagecurrent between the first data line and the second data line.
 26. Themethod of claim 24 further comprising the step of selecting a distancebetween the first pixel electrode and the first data line to provide anacceptable leakage between the first pixel electrode and the first dataline.
 27. The method of claim 24 further comprising the steps of:providing a second pixel electrode of an electronic display incommunication with the source electrode of the second transistor;providing a second data line of the electronic display in communicationwith the drain electrode of the second transistor; and selecting atleast one of: (i) a distance between the source electrode of the firsttransistor and the drain electrode of the first transistor; (ii) achannel width of the first transistor; (iii) a dimension of the firstpixel electrode; (iv) a distance between the first data line and thefirst transistor and (v) a distance between the first pixel electrodeand the second data line, to provide an acceptable leakage currentbetween the first data line and the second data line.
 28. The method ofclaim 19 wherein the step of forming the unpatterned silicon layercomprises forming an amorphous silicon film.
 29. The method of claim 28wherein the step of forming the unpatterned silicon layer comprisesforming an intrinsic amorphous silicon film.
 30. The method of claim 19wherein the steps of forming include mask steps consisting of a firstmask step and a second mask step, wherein the step of forming at leastone gate electrode comprises the first mask step and the steps offorming at least one patterned drain electrode and forming at least onepatterned source electrode share the second mask step.